Most existing broadband wireless standards adopt some form of Orthogonal Frequency Division Multiplexing (also referred to as ‘OFDM’) as a transmission scheme. OFDM is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, DSL broadband internet access, wireless networks, and 4G mobile communications.
The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters Channel equalization is simplified because OFDM may be viewed as using many slowly modulated narrowband signals rather than one rapidly modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to eliminate intersymbol interference (ISI) and utilize echoes and time-spreading (on analogue TV these are visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system. OFDM is disclosed at the website http://en.wikipedia.org/w/index.php?title=Orthogonal_frequency-division_multiplexing&oldid=546868144 and is incorporated herein in its entirety for reference purposes.
Traditionally, OFDM based wireless transceivers used the super heterodyne architecture which requires several analog components (filters and amplifiers) to achieve acceptable signal quality while increasing the overall power consumption and cost considerably. To overcome this drawback, the direct-conversion architecture where the radio frequency (RF) signal is converted directly to baseband, thus eliminating the bulky band-pass surface acoustic wave (SAW) filters, has gained increased popularity recently because it enables low-cost low-power integration in complementary metal oxide semiconductor (CMOS) technology leading to a smaller form factor. However, direct conversion OFDM-based broadband wireless transceivers suffer from several performance-limiting RF/analog impairments including Inphase/Quadrature imbalance (hereinafter referred to as ‘I/Q imbalance’). As used herein, I/Q imbalance refers to the amplitude and phase mismatches between the in-phase (I) and quadrature (Q) branches at the transmit and receive sides. The I/Q imbalance is result of the impairments due to the front end analog parts of the transceivers. I/Q imbalance can degrade the performance of OFDM systems significantly.
Specifically, in direct-conversion transceivers, I/Q modulation and demodulation are performed in the analog domain. Ideally, I and Q branches of the mixers should have equal amplitude and 90 degrees phase shift but this is rarely the case in practice which results in inter-carrier interference (ICI) between the OFDM subcarriers. In addition, mismatches between the low-pass filters in I and Q branches result in FD I/Q imbalance.
Accordingly, there is a need for an improved system and a method that can compensate performance limiting RF/analog impairments including I/Q imbalance in beamforming OFDM systems in a reliable, power efficient and cost efficient manner.